1. Field of the Invention
The present invention relates to an optical head assembly suitable for use with an optical disk apparatus which records and reproduces data from an optical disk as desired.
2. Description of the Prior Art
An optical disk apparatus is available for recording or reproducing information out of an optical disk as desired in response to a command which is fed from a computer or similar processing system, as well known in the art. Generally, such an apparatus is comprised of a drive unit for driving an optical disk in a rotary motion, an optical head, and a control unit for controlling the drive unit and optical head. The operation of an optical disk apparatus is such that a radiation issuing from a semiconductor laser or similar light source in the form of a light beam is focused onto an optical disk to scan minute tracks provided on the disk at the intervals of 1.6 microns or so, whereby information is recorded in or reproduced from the tracks as needed. The optical head, therefore, has to fulfill three different kinds of functions: focus control adapted to regulate the light beam for adequate convergence such as to a spot diameter of 1 micron, track control for preventing the spot from moving out of a desired track, and access control for searching for a desired track.
Referring to FIG. 10 of the drawings, an example of prior art optical heads is schematically shown. As shown, the optical head includes optics which is made up of a semiconductor laser or similar light source 51, a beam splitter 52, and an objective lens 53. Also included in the optical head are a twin-element photodiode 54 and a magnetic circuit block 55 which is adapted to selectively shift the objective lens 53 in a focusing direction F and a track traversing direction T. The focusing direction F is perpendicular to a recording medium surface LDa of an optical disk LD while the track traversing direction T is parallel to the radial direction of the optical disk LD.
In operation, a light beam 56 issuing from the light source 51 is transmitted through the beam splitter 52 and then focused by the objective lens 53 onto a particular point P.sub.1 on the disk LD. The resulting reflection 56a from the disk LD is transmitted through the objective lens 53 and thereby expanded in beam diameter. This expanded light beam is reflected and deflected perpendicularly by the beam splitter 52 to become incident to a photosensitive surface 57 of the twin-element photodiode 54. The photosensitive surface 57 of the twin-element photodiode 54 is divided into a pair of photosensitive areas 57a and 57b so that a tracking error may be detected in terms of a difference between the intensity of light incident to the photosensitive areas 57a and that of light incident to the photosensitive area 57b. A signal representative of such a tracking error is used to control the magnetic circuit block 55 and thereby to shift the objective lens 53 in the track traversing direction T. The parts and elements described so far implement the previously mentioned track control.
Focusing error detecting means produces a signal representative of a focusing error, although not shown in the figure. The magnetic circuit block 55 is controlled by the focusing error signal to move the objective lens 53 in the focusing direction F, whereby the focus control is accomplished.
A drawback with the tracking control system discussed above is that upon the shift of a track position being scanned from the point P.sub.1 to a point P.sub.2 a reflection 56b from the point P.sub.1 to a point P.sub.2 a reflection 56b from the point P.sub.2 is brought out of the optical axis of the objective lens 53, as indicated by a dash-and-dot line in FIG. 10. In this condition, the beam incident to the photosensitive surface 57 of the twin-element photodiode 54 is shifted in proportion to the displacement of the objective lens 53, introducing an offset in the tracking error signal. It follows that the movable range of the objective lens 53 in the track traversing direction T cannot exceed 100 microns or so which accommodates the offset. Hence, the access control which is far broader than the track control as to the movable range of the lens 53 in the track traversing direction T has to be effected by driving the optical head assembly by a motor or similar transporting device. Then, the access speed would be critically effected by the mass or weight of the optical head assembly.
An approach heretofore proposed for providing faster access is to construct the object lens 53, magnetic circuit block 55 and beam splitter 52 to be movable while fixing the other optical elements in place. This kind of scheme may be successful in reducing the weight of the movable sections to a significant degree, compared to the case wherein the entire optical head is movable. However, since the magnetic circuit block 55 which is one of the movable sections includes a permanent magnets, yokes and other various members, an access time several times longer than with the head assembly for a magnetic disk is needed.
FIG. 11 shows another prior art optical head assembly which is also directed toward faster access and disclosed in Japanese patent laid-open publication No. 11544/1984. In the FIG. a magnetic circuit block 62 is fixed in place to extend in the track traversing direction T and provided with two magnetic gaps 61 which individually cover the entire track extension. The magnetic circuit block 62 includes permanent magnets 63 and yokes 64. A pair of flat support pieces 65 are positioned in such a manner as to sandwich the magnetic circuit block 62 from above and below. An objective lens 66 and a movable coil 67 are securely mounted on the support pieces 65.
The upper and lower support pieces 65 are mounted on two parallel frames 69 through rollers 68 which are located at four corners of each support piece 65, whereby the support pieces 65 are movable up and down, i.e., in the focusing direction F. The frames 69 in turn are mounted through rollers 71 on parallel guide rails 72 which are fixed to the magnetic circuit block 62 and, therefore, the frames 69 are movable in the lengthwise direction T of the block 62. That is, the optical head assembly of FIG. 11 is constructed and arranged such that the support plates 65 carrying the objective lens 66 therewith is selectively movable in the focusing direction F and the track traversing direction T while traversing all of the tracks of an optical disk in the traversing direction T, due to electromagnetic forces exerted by the focusing coil and tracking coil.
It will therefore be seen that the optical head assembly of FIG. 11 is effective to reduce the weight or mass of the movable section because the magnetic circuit block 62 having a substantial weight is not movable. Nevertheless, this kind of scheme has a shortcoming that the movable section for driving the objective lens 66 in the focusing direction F and the track traversing direction T has a complicated structure and therefore involves a great number of structural parts and elements. Another shortcoming is that the mechanical strength or rigidity of the movable section is not sufficient.
Generally, thrust developed by the electromagnetic force of a coil which is disposed in a magnetic gap is proportional to the flux density, and the acceleration of a movable section is produced by dividing the thrust by the weight of the movable section. The access time is proportional to the square root of the acceleration. With the optical head assembly of FIG. 11, it is impractical to increase the flux density in the magnetic gap 61 to a satisfactory degree because increasing it to such a degree would cause the magnetic flux to concentrate on end portions 64a of the yokes 64 to bring about magnetic saturation. For this reason, it is impossible to apply a sufficient thrust to the support pieces 65.
The yokes 64 may be provided with a thicker configuration to increase the flux density. This, however, requires the frames 69 and movable coil 67 to be increased in width also, resulting in an increase in the overall weight of the assembly. Moreover, broader dimensions of the support pieces 65 and frames 69 aggravate the mechanical strength or rigidity of the same and thereby lowers the structural resonance frequency of the movable section. Usually, a servo band of several kilohertz is needed for the focus control and track control of an optical head, as generally accepted. Hence, the structural resonance frequency of the movable section has to be selected to be at least five times higher than the servo band. Lower resonance frequencies of the movable band, therefore, would limit the servo band available and degrade the following characteristic. In addition, the mechanical strength of the support pieces 65 and frames 69 cannot be increased without increasing the weight of the movable section and therefore without limiting the access speed. Consequently, it is impossible to increase the access time to a sufficient degree.